Deuterium commonly refers to the hydrogen atom with one neutron, while triton refers to the hydrogen nucleus with two neutrons, matching atom and nucleus of hydrogen respectively.

Planck's constant has units of action (energy × time), which are equivalent to angular momentum.

Electron count sums to 18 (so Z=18 protons); with A=40, neutrons = 40-18 = 22, so option A (22 neutrons, 18 protons) is correct.

Overlap of valence and conduction bands allows free carrier motion characteristic of metals.

The n=3 to n=2 transition in hydrogen corresponds to H-alpha with wavelength ~656 nm, giving frequency ≈ 4.6 × 10¹⁴ Hz.

Nuclear fusion is the process where two light nuclei combine to form a heavier nucleus, releasing energy.

Ionisation energy from level n is 13.6/n^2; for n = 2 this gives 13.6/4 = 3.4 eV, so option C is correct.

In the context of photoelectric effect apparatus, symbols like I1 and I2 typically denote measured photoelectric currents under different conditions.

Pair production converts photon energy into particle pairs, i.e., it materializes energy into matter.

The OCR header indicates option A (2,182 me V) as the rest mass given for Aº in the text.